Ketoisophorone (4-oxoisophorone), a useful intermediate for a starting material of medicines, perfumes, condiments, and for polymer, is produced from isophorone and the like. For example, as a process for producing 4-oxoisophorone by oxidizing .alpha.-isophorone with oxygen, there have been proposed a method in which .alpha.-isophorone is oxidized with oxygen in the presence of a phosphomolybdic acid or a silicomolybdic acid [Japanese Patent Publication No. 30696/1980 (JP-B-55-30696)], a method in which a-isophorone is oxidized with oxygen in the coexistence of a phosphomolybdic acid or a silicomolybdic acid and an alkaline metal compound or an aromatic amine [Japanese Patent ApplicationLaid-OpenNo.191645/1986 (JP-A-61-191645)], and a method in which a-isophorone is oxidized with oxygen in the presence of a vanadium catalyst [Japanese Patent Application Laid-Open No. 93947/1975 (JP-A-50-93947)]. Japanese Patent Application Laid-Open No. 81347/1974 (JP-A-49-81347) discloses a method for producing 4-oxoisophorone by oxidizing a-isophorone with an alkaline metal chromic acid salt or a dichromate or a chromium trioxide. In the Chem. Lett. (1983), (7), 1081, there is disclosed a method for producing 4-oxoisophorone by oxidizing a-isophorone using t-butylhydroperoxide in the presence of a palladium catalyst. However, in these methods, the selectivity of ketoisophorone is reduced, therefore separation of the formed by-product(s) or a metal catalyst and purification of the object compound are complicated. Moreover, these methods involve using a heavy metal compound requiring special treatment, such as chromium, or a peroxide needed to be handled with care, which results in a decrease in working efficiency.
Moreover, as a method for producing ketoisophorone from .beta.-isophorone, Japanese Patent Application Laid-Open No. 125316 (JP-A-51-125316) discloses a method for producing an ethylenically unsaturated dicarboxylic acid by oxidizing .beta.-ethylenically unsaturated ketone with molecular oxygen or a molecular oxygen-containing gas in the presence of an inorganic base or an organic base and a cobalt or manganese chelate. In this method, however, the yield of ketoisophorone is low due to the use of a straight-chain secondary or tertiary amine such as triethylamine as the organic base.
In Japanese Patent Application Laid-Open No. 53553/1998 (JP-A-10-53553) discloses a method for producing ketoisophorone by oxidizing .beta.-isophorone with molecular oxygen in the presence of bis(2-hydroxybenzylidene)ethylenediamine-manganese complex salt (i.e., manganese-salene), an organic base, a specific substance having a catalytic action (e.g., acetylacetone), and water. In the literature, there is recited as the manganese complex salt a complex in which 1 mole of bis(2-hydroxybenzylidene)ethylenediamine is coordinated relative to 1 mole of manganese. However, even in the above method using the above manganese complex salt, the conversion and the selectivity of a substrate are not improved enough. Particularly, a higher concentration of .beta.-isophorone in the reaction system causes a considerable decrease in the yield of ketoisophorone. For example, when the concentration of .beta.-isophorone is 20% by weight or more, the conversion and/or the selectivity is decreased to a large extent. Therefore, relatively large amounts of a manganese complex salt and an organic base are required for an improved conversion. Further, a lower concentration of oxygen remarkably decreases the reaction rate.